In recent years, with global environment getting worse, much attention is increasingly paid to recycling of resins and to additives which are safe for living bodies and less harmful to global environment. Resins represented by polylactic acids (PLA), polyglycolic acids (PGA), polycaprolactones (PCL), and the like are utilized as biodegradable resins which can be degraded by water or an enzyme under a natural environment or in a living body.
For example, since the PLA has good processability and the molded article has excellent mechanical strength, it is utilized as disposable containers, packages, and the like. However, since the PLA has relatively slow degradation speed under a condition other than in compost (for example: in sea water, soil, or the like), it can scarcely be used for a purpose in which it is required to degrade and vanish the PLA within several months. Also, in the case of using a PLA for a slow-release medicine, the PLA has slow degradation speed in a living body, and it remains in the living body for a long time after the agent has been released. Thus, it cannot sufficiently respond to the needs of medicines in which the agent is slowly released within a relatively short period.
As a method for improving hydrolysis speed of a PLA to overcome the problem, for example, a method by compounding a hydrophilic additive such as a polyethylene glycol with the PLA is proposed. However, the PLA is less hydrophilic and is hardly compatible to a hydrophilic additive such as a polyethylene glycol. Therefore, the additive bleeds out during molding or after molding, the mechanical strength of the molded article is decreased, and the appearance such as transparency is deteriorated. In consequence, it is not practical.
Also, in order to improve hydrolysis speed of a biodegradable resin such as a PLA, a resin composition, which has a constituent unit derived from aspartic acid or the like and a constituent unit derived from lactic acid or the like as a degradation accelerator with a biodegradable resin such as a PLA, is provided (Patent Document 1). This resin composition shows sufficient hydrolyzability depending on purpose. However, with diversification of the consumer's needs, the needs to a biodegradable resin are sophisticated, and a resin composition having a high degradation speed and higher transparency is required depending on purpose.
Also, it is known to use a malic acid saturated ester compound or a citric acid saturated ester compound as a plasticizer to give flexibility to a polylactic acid (Patent Document 2 and Patent Document 3), and it is also known that, in the case of adding a large amount of a citric acid saturated ester compound, hydrolysis of the polylactic acid occurs (Non-Patent Document 1). However, in the case of using a small amount of such a plasticizer, hydrolyzability is not developed while, in the case of using a large amount of it, the mechanical strength and heat resistance of the molded article are largely decreased. In consequence, it is not practical.